The New England Brumby Sanctuary horses have been given the jab in a bid to make them infertile, according to founder Jan Carter.

But it has so far only reduced their sperm count.

“We will know more about the effectiveness of the drug towards the end of the year after the horses have gone through their main cycling season,” Ms Carter said.

It has been about six weeks since the trial on wild horses started at the sanctuary.

Mares were intially treated with  the contraceptive drug, porcine zona pellucide, or PZP, in the Australian-first trial.

So far there have been no ill-effects.

The next step in the trial is to give stallion brumbies the jab to render them infertile.

But early results indicate the drug  has only reduced the sperm count of the male horses rather than rendering them totally infertile.

But Ms Carter said it could be enough to help reduce fertility rates among wild stallions in an effort of controlling numbers.

The real test will come in spring.

“The mares are not cycling now and we have some stallions in there with them,” Ms Carter said.

PZP has successfully been used to control mustang populations in the United States, with the drug’s effects lasting for about four years.

Ms Carter said the beauty of the drug was it was manufactured in Australia and was easy to apply.

“It is quite simple; just a two mil injection in the rump and then they are good to go.

“It takes two seconds.”

The hope for the sanctuary is to introduce birth control of brumbies as a way to control wild populations in national parks, rather than culling.

“The drug is cheaper than a bullet,” Ms Carter said.

“The government is always looking for a quick fix but if you are shooting them, you have to trap them first anyway.

“It makes much more sense to treat them, tag them and then let them go because then you are also keeping the strength in their genetics.” If the trial is successful, Ms Carter believes the wide-scale treatment of wild brumbies could be introduced within the next 10 to 15 years.